Liquid crystals esters

ABSTRACT

A liquid crystal ester has a structure: ##STR1## where X and Y are fluorine or hydrogen at least one being hydrogen, and Z is either: 
     (a) ##STR2## where R 2  is alkyl, alkoxy, alkylcarbonyloxy or alkoxycarbonyloxy 
     (b) ##STR3## where R 3  is an alkyl group. The ester may be prepared by a conventional esterification from the corresponding phenol ##STR4## and the corresponding acid chloride Z--COCl. The ester may be used in mixtures with other liquid crystal compounds for use in known electro-optic devices.

The present invention relates to liquid crystal esters and materials anddevices containing them.

The use of liquid crystal materials to exhibit electro-optic effects indisplay devices such as digital calculators or watches is now wellknown. One of the parameters of a liquid crystal material which isimportant in relation to electro-optical operation is the dielectricanisotropy (Δε) of the material. This is the difference, for a givenfrequency and temperature, between the average dielectric constantmeasured parallel (ε.sub.∥) to the long axis of the molecules of thematerial, eg when aligned together, less the average dielectric constantmeasured perpendicular (ε.sub.⊥) to the long axis of the molecules.

The sign and magnitude of the dielectric anisotropy of a given liquidcrystal material is one of the major parameters which determine thekinds of electro-optic devices in which that material may be used.

For example, materials having a large positive dielectric anisotropy,herein referred to as `strongly positive` materials may be used in knowntwisted nematic effect devices. For example mixtures of 4-alkyl- or-alkoxy-4'-cyanobiphenyls and a 4"-alkyl- or -alkoxy-cyano-p-terphenylhave achieved considerable commercial success in such applications.

Materials having a large negative dielectric anisotropy may for example(depending on other properties such as resistivity) be used in knowndynamic scattering effect and cholesteric memory effect devices.

Materials having a small positive or negative dielectric anisotropy,herein referred to as `weakly` positive or negative materials asappropriate, may be mixed with either strongly positive materials orstrongly negative materials to modify the dielectric anisotropy.

For example weakly positive or negative materials are useful forblending with strongly positive materials for multiplexed twistednematic effect devices. Furthermore, they may also be blended withstrongly negative materials to form suitable mixtures for positivecontrast guest-host devices. They may also be used in other knownapplications.

It is the purpose of the present invention (in one aspect) to provideliquid crystal compounds which are either weakly positive or weaklynegative materials (preferably negative).

The sign and magnitude of the dielectric anisotropy of a liquid crystalmaterial is determined (inter alia) by the resultant dipole moment ofthe molecules of the material which in turn is determined by the varioussubstituents within the molecule. Positive materials have a resultantdipole moment along the long axis of the molecule, whilst negativematerials possess a resultant dipole moment at right angles to the longaxis of the molecule.

The known group of liquid crystals based on the ester structure (I) asgiven below in which the six membered ring A may be aromatic orsaturated and in which both rings will carry appropriate substituents inthe 4 and 4' positions, ##STR5## may show positive or negativedielectric anisotropy according to the degree of interactions of the 4and 4' substituents with the dipole produced by the central carbonylgrouping.

Known weakly positive and negative esters having the structure as instructure (I) above are those in which the ring A is a benzene orcyclohexane ring and the substituents in the 4 and 4' positions arealkyl groups. These known esters, like many other liquid crystalmaterials, exhibit smectic liquid crystal phases or show tendencies toproduce such phases at temperatures below their nematic ranges. Suchsmectic properties are undesirable particularly in applications wherethe materials are used in mixtures, eg for multiplexed devices asdescribed in published UK Patent Application No. 78.36305, because theycan give rise to large `injected` smectic areas on the mixturecomposition phase diagram (composition plotted against temperature).Such areas can limit the working composition range on the phase diagramlimiting the choice of compositions which may be employed in devices andconsequently the ability of the device designer to adjustelectro-optical properties by varying mixture composition.

It is an object of the present invention to provide weakly positive ornegative esters showing, in general terms, reduced smectic behaviour ascompared with the known esters mentioned above.

According to the present invention there is provided a liquid crystalester compound having the following molecular structure: ##STR6## whereX and Y are fluorine or hydrogen at least one being fluorine, where R₁is an alkyl group and Z is either:

(a) ##STR7## where R₂ is an alkyl, alkoxy, alkylcarbonyloxy oralkoxycarbonyloxy group

(b) ##STR8## where R₃ is an alkyl group.

Preferably X=H and Y=F.

A compound (or compounds) having a molecular structure as in formula(II) will be referred to as a compound (or compounds), as defined.

The groups R₁, R₂ and R₃ preferably contain less than eighteen carbonatoms, eg one to ten carbon atoms, and may be normal or branched. If oneof these groups is branched it may contain a chiral centre in which casethe compound is optically active.

Preferably Z in formula (II) is either ##STR9## where R₃ ' and R₃ " arealkyl groups.

By a `liquid crystal compound` is meant a compound in one of thefollowing two known categories:

(i) Compounds which normally exhibit a liquid crystal phase.

(ii) Compounds which do not normally exhibit a liquid crystal phase butwhich nevertheless usefully affect some aspect of liquid crystalbehaviour when dissolved in other liquid crystal compounds.

Compounds in category (ii) show a `monotropic` or virtual liquid crystalto isotropic liquid transition at a temperature below the melting pointof their solid phase. The monotropic or virtual transition may bedetected respectively by rapid cooling of the liquid phase or bydissolving the compound in a material exhibiting a liquid crystal phase,observing the change in the transition to the isotropic liquid phase ofthe material by the addition and calculating the virtual transitiontemperature by extrapolation.

Compounds in category (ii) might for example be usefully dissolved inother liquid crystal compounds to extend or vary the liquid crystaltemperature ranges of the compounds or to vary the molecular helicalpitch (in the case of `cholesteric` liquid crystals).

Material which exhibits a liquid crystal phase and either consists of orcontains a compound as defined above will be referred to herein as amaterial `as defined`.

The liquid crystal phase exhibited by the material as defined may benematic or cholesteric although the nematic phase is likely to be themost useful.

For a nematic phase the compound as defined should preferably have anormal alkyl group R₁ as well as a normal group R₂ or R₃ whilst for acholesteric phase the compound as defined should preferably have abranched alkyl group containing a chiral centre in at least one of thegroups R₁ and R₂ or R₃.

Compounds having formula (II) have been found to show, in general,reduced smectic tendencies, as compared with the known esters mentionedabove.

The fact that compounds as defined (II) are liquid crystal (see below)is unexpected because the introduction of lateral substituents into themolecular structure usually diminishes the lateral attractions betweenthe molecules which produce liquid crystal properties.

Preferably compounds as defined are prepared by reacting the acidchloride Z--COCl with the phenol ##STR10## in the case where ##STR11##where R₂ is an alkyl or alkoxy group or ##STR12## The reaction may becarried out under conditions well known for esterification reactions.

A compound as defined may be mixed with one or more other compounds asdefined to enhance its liquid crystal properties, eg to extend itsliquid crystal temperature range or otherwise to modify the properties,eg the electrical properties of the liquid crystal phase in anadvantageous manner.

For example the following are examples of such mixtures: ##STR13## whereR₄ to R₁₈ are various n-alkyl groups containing ten or less carbonatoms.

Likewise, compounds as defined may be mixed with one or more of thecompounds in the following known categories: ##STR14## or knownlaterally substituted derivatives of these compounds, where ##STR15## isa cyclohexane ring, ##STR16## is a bicyclo(2.2.2)octane ring, ##STR17##and Y₁ is CN, or R¹, or OR¹ or CO.O--X--Y¹ where Y¹ is CN, or R² or OR³; the definition of R² is the same as that used earlier for R₁.

According to another aspect of the invention, an electro-optic liquidcrystal device includes two adjacent plates at least one of which isoptically transparent, a layer of liquid crystal material contained inthe space between the plates wherein the liquid crystal material is thematerial as defined above, and deposited on the inner facing surfaces ofthe plates, electrodes to allow an electric field to be applied acrossthe layer. The electro-optic device may for example be a display deviceused, for instance, in an instrument such as a time watch or clock. Thedevice may be one of the following known kinds: twisted nematic effect;(cholesteric-to-nematic) phase change effect; Freedericksz effect;dynamic scattering effect; or cholesteric memory effect. It is wellknown to those skilled in the art how materials may be used in thesevarious applications and it will be apparent to such persons how toprepare suitable materials depending on the applications.

Compounds as defined above are, in general, reasonably chemically andphotochemically stable liquid crystal compounds which have a lowdielectric anisotropy and low birefringence and, in the case where##STR18## low viscosity.

The low birefringence makes the compounds as defined particularly usefulfor blending with other liquid crystal materials which together give along helical pitch cholesteric material for use in(cholesteric-to-nematic) phase change effect devices (of either positiveor negative contrast as explained below).

The small dielectric anisotropy makes the compounds as defined usefulfor blending with other materials to form mixtures having a reduced(positive or negative) dielectric anisotropy. Examples of materials anddevices in which the compounds as defined may be used in this way areany of those given above. It is well known to those skilled in the artthat the dielectric anisotropy of a mixture can be controlled byappropriate control of the relative proportions of the materials blendedtogether to form the mixture.

The compounds as defined are particularly suitable for use in mixtureswith positive dielectric anisotropy materials, eg cyanobiphenyls, foruse in multiplexed twisted nematic devices. This use is furtherdescribed in a copending UK Patent Application No 7934125.

Examples of the preparation and properties of compounds as defined abovewill now be given.

EXAMPLE 1

Preparation of compounds as defined above wherein ##STR19## R₂ '=alkylor alkoxy.

These compounds are preferably prepared by the esterification of theacid chlorides of the appropriate 4-n-alkyl- and -alkoxybenzoic acids(known) or trans-4-n-alkylcyclohexane-1-carboxylic acids (synthesised bystandard synthetic methods) as appropriate together with the appropriate4-n-alkyl-2- or -3-fluorophenols which are prepared (as described incopending UK Patent Application No 7934128) as follows: ##STR20## whereR, R'" and R₁ are alkyl groups.

Thus the reaction pathway is preferably: ##STR21## where B is atrans-cyclohexane ring, R₁ being n-alkyl, or where B is a phenyl ring,R₂ being n-alkyl or n-alkoxy.

The preparation of 4-n-pentyl 1-2-fluorophenyl 4'-n-butyloxybenzoatetypifies the esterification procedure used to prepare these esters. Thiscompound has been produced as follows.

STEP A1

4-n-butyloxybenzoic acid (0.0082M) was converted into the acid chlorideusing freshly distilled thionyl chloride (15 cm³).

STEP B1

To a cold, stirred solution of the acid chloride prepared in Step A1 indry dichloromethane (15 cm³) was added, dropwise, a cold solution of4-n-pentyl-2-fluorophenol (0.0082M) in dry dichloromethane (15 cm³) andtriethylamine (10 cm³), keeping the temperature below 10° C. When theaddition had been completed, the reaction mixture was heated underreflux, with stirring for 3 hr.

After cooling, the dichloromethane and triethylamine were distilled off,under reduced pressure, and ether was then added to the residue. Theundissolved triethylamine hydrochloride was filtered off and theethereal extract washed successively with 10% hydrochloric acid, 5%aqueous sodium carbonate, and water and finally dried (Na₂ SO₄).

The crude residual ester was then column chromatographed on silica gel,eluting with chloroform (2): light petroleum (bp 60°-80°) (1). Severalrecrystallisations from ethanol gave the pentyl-2-fluorophenyl4'-n-butyloxybenzoate as a colourless crystalline solid, 1.9 g (61%yield).

Esters such as 2-fluoro-4-n-pentylphenyl 4'-n-butylbenzoate and2-fluoro-4-n-pentylphenyl trans-4'-n-butylcyclohexylcarboxylate werepurified by the above chromatographic procedure and then furtherpurified by distillation under reduced pressure, to afford the esters ascolourless liquids. The cyclohexane ester mentioned above was furtherrecrystallised from ethanol, using solid carbon dioxide, to give thisester as a colourless crystalline solid, which rapidly melted at roomtemperature.

Some products prepared by the above method, together with their liquidcrystal transition temperature, are listed in Tables 1-4 below.

Spectroscopic analysis, thin layer chromatography, and gas liquidchromatography were carried out on all the esters prepared by the methodto verify their structure and/or purity.

Examples of products of Step B1 are given in Tables 1-4 below.

In the Tables the following symbols are used:

N-I=nematic to isotropic liquid transition temperature

C-N=crystalline solid to nematic transition temperature

C-I=crystalline solid to isotropic liquid transition temperature

C-Ch=crystalline solid to cholesteric transition temperature

Ch-I=cholesteric to isotropic liquid transition temperature

C-S_(A) =crystal to smectic A transition temperature

S_(A) -N=smectic A to nematic transition temperature

S_(B) -S_(A) =smectic B to smectic A transition temperature

S_(A) -I=smectic A to isotropic liquid transition temperature

( )=monotropic transition temperature

[ ]=virtual transition temperature

(+)-=optically active compound having a positive optical rotation angle

                  TABLE 1                                                         ______________________________________                                         ##STR22##                                                                     ##STR23##                                                                    cyclohexane ring)                                                                                 C-N or                                                                        C-S.sub.A                                                                     or (°C.)                                                                        N-I   Ch-I  S.sub.A -N                           R.sub.3                                                                              R.sub.1      C-I      (°C.)                                                                        (°C.)                                                                        (°C.)                         ______________________________________                                        CH.sub.3                                                                             n-C.sub.3 H.sub.7                                                                          27       [-27]                                            CH.sub.3                                                                             n-C.sub.5 H.sub.11                                                                         13.5     [-36]                                            CH.sub.3                                                                             n-C.sub.7 H.sub.15                                                                         18.5     [-24]                                            C.sub.2 H.sub.5                                                                      n-C.sub.5 H.sub.11                                                                         23       [-10]                                            C.sub.2 H.sub.5                                                                      n-C.sub.7 H.sub.15                                                                         13       (-5.5)                                           n-C.sub.4 H.sub.9                                                                    n-C.sub.3 H.sub.7                                                                          10.5     10.9                                             n-C.sub.5 H.sub.11                                                                   n-C.sub.4 H.sub.9                                                                          10       23                                               n-C.sub.5 H.sub.11                                                                   n-C.sub.6 H.sub.13                                                                         10       32.8                                             n-C.sub.5 H.sub. 11                                                                  n-C.sub.8 H.sub.17                                                                         25       39.4                                             n-C.sub.5 H.sub.11                                                                   n-C.sub.9 H.sub.19                                                                         38       43.8                                             n-C.sub.5 H.sub.11                                                                   (+)-2 methylbutyl                                                                          38             [-1]                                       n-C.sub.6 H.sub.13                                                                   n-C.sub.3 H.sub.7                                                                          22       29.6                                             n-C.sub.6 H.sub.13                                                                   n-C.sub.5 H.sub.11                                                                         11.5     34.3                                             n-C.sub.6 H.sub.13                                                                   n-C.sub.6 H.sub.13                                                                         11       30.7                                             n-C.sub.6 H.sub.13                                                                   n-C.sub.7 H.sub.15                                                                         22       38.9                                             n-C.sub.6 H.sub.13                                                                   n-C.sub.8 H.sub.17                                                                         16       36.2                                             n-C.sub.6 H.sub.13                                                                   n-C.sub.9 H.sub.19                                                                         28.5     43.0                                             n-C.sub.7 H.sub.15                                                                   n-C.sub.3 H.sub.7                                                                          30.5     39.2                                             n-C.sub.7 H.sub.15                                                                   n-C.sub.4 H.sub.9                                                                          14.5     24.1                                             n-C.sub.7 H.sub.15                                                                   n-C.sub.5 H.sub.11                                                                         24.5     43.4                                             n-C.sub.7 H.sub.15                                                                   n-C.sub.6 H.sub.13                                                                         18.5     37.8                                             n-C.sub.7 H.sub.15                                                                   n-C.sub.7 H.sub.15                                                                         30       46.4                                             n-C.sub.7 H.sub.15                                                                   n-C.sub.8 H.sub.17                                                                         33       45.0                                             n-C.sub.7 H.sub.15                                                                   n-C.sub.9 H.sub.19                                                                         37       49.3                                             n-C.sub.7 H.sub.15                                                                   n-C.sub.11 H.sub.23                                                                        42.5     52          47                                   n-C.sub.7 H.sub.15                                                                   (+)-2 methylbutyl                                                                          26.5           (12.1)                                     n-C.sub.8 H.sub.17                                                                   n-C.sub.3 H.sub.7                                                                          21.5     38.5                                             n-C.sub.8 H.sub.17                                                                   n-C.sub.5 H.sub.11                                                                         23.5     41.0                                             n-C.sub.8 H.sub.17                                                                   n-C.sub.7 H.sub.15                                                                         22.5     44.0                                             n-C.sub.9 H.sub.19                                                                   n-C.sub.5 H.sub.11                                                                         41.0     47.7                                             n-C.sub.9 H.sub.19                                                                   n-C.sub.7 H.sub.15                                                                         34.0     49.3                                             n-C.sub.3 H.sub.7                                                                    C.sub.3 H.sub.7                                                                            32.5     (19)                                             n-C.sub.3 H.sub.7                                                                    n-C.sub.5 H.sub.11                                                                         36       (26.5)                                           n-C.sub.3 H.sub.7                                                                    n-C.sub.7 H.sub.15                                                                         27       30.3                                             n-C.sub.4 H.sub.9                                                                    n-C.sub.5 H.sub.11                                                                         10.5     16.1                                             n-C.sub.4 H.sub.9                                                                    n-C.sub.7 H.sub.15                                                                         16       26.7                                             n-C.sub.5 H.sub.11                                                                   C.sub.3 H.sub.7                                                                            30       30.7                                             n-C.sub.5 H.sub.11                                                                   C.sub.5 H.sub.11                                                                           17.5     35.0                                             n-C.sub.5 H.sub.11                                                                   C.sub.7 H.sub.15                                                                           33       41.4                                             ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                        Compounds of the form                                                          ##STR24##                                                                                        C-N or                                                                        C-I or C-Ch N-I    Ch-I                                   R.sub.2                                                                              R.sub.1      (°C.)                                                                              (°C.)                                                                         (°C.)                           ______________________________________                                        CH.sub.3 O                                                                           n-C.sub.5 H.sub.11                                                                         28          (20)                                          n-C.sub.4 H.sub.9 O                                                                  n-C.sub.5 H.sub.11                                                                         44          (37.9)                                        n-C.sub.6 H.sub.13 O                                                                 n-C.sub.5 H.sub.11                                                                         28          37.0                                          n-C.sub.3 H.sub.7                                                                    n-C.sub.7 H.sub.15                                                                         23          [-7]                                          n-C.sub.4 H.sub.9                                                                    n-C.sub.5 H.sub.11                                                                         16          [-14]                                         n-C.sub.5 H.sub.11                                                                   n-C.sub.5 H.sub.11                                                                         14          (1.2)                                         n-C.sub.5 H.sub.11                                                                   n-C.sub.7 H.sub.15                                                                         24          (4.2)                                         n-C.sub.5 H.sub.11                                                                   n-C.sub.3 H.sub.7                                                                          7.0         [<-20]                                        n-C.sub.5 H.sub.11                                                                   (+)-2-methylbutyl                                                                          1.5                4.7                                    n-C.sub.6 H.sub.13                                                                   n-C.sub.5 H.sub.11                                                                         19.5        [-5]                                          n-C.sub.7 H.sub.15                                                                   n-C.sub.5 H.sub.11                                                                         19.0        (4.3)                                         n-C.sub.7 H.sub.15                                                                   (+)-2-methylbutyl                                                                          27.0               (14.1)                                 ______________________________________                                    

                  TABLE 3                                                         ______________________________________                                        Compounds of the form                                                          ##STR25##                                                                                     C-I (°C.)                                             R.sub.2                                                                              R.sub.1   or C-N    N-I (°C.)                                                                     C-S.sub.A                                                                           S.sub.A -I                            ______________________________________                                        n-C.sub.4 H.sub.9                                                                    n-C.sub.5 H.sub.11                                                                       2.7      [<-9.0]                                            n-C.sub.5 H.sub.11                                                                   n-C.sub.5 H.sub.11                                                                      15.0      (-8.0)                                             CH.sub.3 O                                                                           n-C.sub.4 H.sub.9                                                                       51.5                                                         n-C.sub.6 H.sub.13 O                                                                 n-C.sub.5 H.sub.11         39.5  48.6                                  n-C.sub.5 H.sub.11                                                                   n-C.sub.3 H.sub.7                                                                       33.5      [14]                                               ______________________________________                                    

                  TABLE 4                                                         ______________________________________                                        Compounds of the form                                                          ##STR26##                                                                                     C-I    S.sub.A -N                                                                          S.sub.B -S.sub.A                                                                    S.sub.A -I                                                                          N-I                                 R.sub.3 R.sub.1  (°C.)                                                                         (°C.)                                                                        (°C.)                                                                        (°C.)                                                                        (°C.)                        ______________________________________                                        n-C.sub.3 H.sub.7                                                                     n-C.sub.5 H.sub.11                                                                     26.5                     (9.2)                               n-C.sub.4 H.sub.9                                                                     n-C.sub.5 H.sub.11                                                                     14.5   (-3.7)            (7.4)                               n-C.sub.5 H.sub.11                                                                    n-C.sub.5 H.sub.11                                                                     27.5    (18.7)           (26.3)                              n-C.sub.6 H.sub.13                                                                    n-C.sub.5 H.sub.11                                                                     29.0               (24.7)                                    n-C.sub.7 H.sub.15                                                                    n-C.sub.5 H.sub.11                                                                     33.5         (21.7)                                                                              34.4                                      n-C.sub.5 H.sub.11                                                                    n-C.sub.3 H.sub.7                                                                      33.5                     [14]                                ______________________________________                                    

EXAMPLE 2

Preparation of compounds as defined above wherein Z is R"₂, R"₂=alkanoyloxy or alkoxycarbonyloxy.

The following route is preferable used for the preparation of thesecompounds: ##STR27## where R_(A) =alkyl.

STEP A2

4-Hydroxybenzoic acid is converted into the hydroxy ester by a method asdescribed by Lowrance (Tetrahedron Lett. 1971, 3453) in which thereactants (4-hydroxybenzoic acid and the appropriate fluorophenol) aredissolved in toluene and heated in a Dean and Stark apparatus togetherwith sulphuric acid and boric acid as catalysts.

STEP B2

The hydroxy ester is converted into the alkanoyl derivative by astandard esterification procedure using the appropriate acid chlorideR_(A) COCl.

STEP C2

The hydroxy ester is converted into the alkyloxy carbonyloxy derivativeby a standard esterification procedure using the appropriate alkylchloroformate ClCOOR_(A).

A liquid crystal device embodying the invention will now be described byway of example only with reference to the accompanying drawings ofwhich:

FIG. 1 is an exploded view of a twisted nematic display;

FIG. 2 is a front view of a watch having a twisted nematic liquidcrystal display constructed as shown in FIG. 1; and

FIG. 3 is a sectional view of the display part of the watch in FIG. 2but drawn to an enlarged scale.

As shown in FIG. 1 a `twisted nematic` type display includes a polariser1 arranged with its polarisation axis 2 vertical. A liquid crystal cell3 comprises two glass slides 4, 5 containing a layer of liquid crystal6, consisting basically of a nematic material which may contain a smallproportion (eg 1%) of a cholesteric compound. Electrodes 7, 8, eg of tinoxide, are arranged on the inner faces of the slides 4, 5. Behind thecell 3 is an analyser or second polariser 11 arranged with itspolarisation axis 12 horizontal. A brushed aluminium reflector 27 isbehind the analyser 11.

Prior to assembling the cell the slides 4, 5 are coated on their innerfaces with silicon monoxide or magnesium fluoride. This coating isformed by evaporating a stream of eg silicon monoxide onto the slide atan angle of about 5° to the surface as for example described in UKPatent Specification No 1,454,296. On assembly the slides are arrangedwith the evaporation direction on two slides 4, 5 at 90° to one another.With such a coating liquid crystal molecules at the coated surfaces liein a single direction (parallel to the evaporation direction) and at anangle of about 25° to 35° typically about 30° to the slide surface. As aresult the liquid crystal molecules progressively twist from one slide 4to the other slide 5 in a twisted homogeneous texture. Thus the plane ofplane polarised light is rotated through 90° on passing through the cell3 providing the direction 13 of the molecules at the surface of theslide 4, 5 is parallel (or perpendicular) to the polarisers axis 2, 12respectively. Thus when zero voltage is applied to the cell light passesthrough the analyser 1, cell 3 and analyser 11, and is then reflectedback to an observer 15.

When a suitable voltage is applied between the electrodes 7, 8 themolecules between the electrodes are caused to align parallel to theapplied field and light transmitted by the polariser is transmittedwithout rotation to the analyser 11. Since the polariser and analyser 11are crossed no light is transmitted to the reflector and no light isreflected back to the observer at those parts of the cell 3 betweenelectrodes 7,8 which therefore appear dark on a lighter background. Thusby shaping each electrode 7, 8 into say seven discrete parts separatelyconnectable to a voltage source the numbers 0 to 9 may be displayed.

In the watch display of FIGS. 2, 3 the electrodes 7, 8 are arranged toprovide four seven bar numeric characters plus a period dot which may bearranged to pulse once per second.

The voltage required to align the molecules parallel to the appliedfield, is the `ON` state, is dependent on layer 6 thickness, materialsused, temperature and surface treatment of the slides 4, 5. A typicalthickness of layer is 12 μm. The material used must be stable over thetemperatures of use ie ambient temperatures which can be assumedconstant for comparison purposes. A surface treatment ie evaporation ofsilicon monoxide at 5° giving large tilt eg 30° of molecules reduces thevoltage necessary to turn the cell `ON` at the expense of a degradedcontrast or speed.

In typical watch displays using twisted nematic liquid crystal displaysa 1.5 volt battery is used to drive the oscillator and logic circuits ofthe watch and an up converter is used to raise the voltage to 3.0 voltsfor operating the display.

Material embodying the invention suitable for use as the liquid crystal6 is:

    __________________________________________________________________________     Mixture B                                                                           ##STR28##                     60% by weight                                   ##STR29##                    39% by weight                                    ##STR30##                     1% by weight                                   (known).                                                                __________________________________________________________________________

Addition of the ester ##STR31## to the biphenyl Mixture B reduces thetemperature dependence of the threshold voltage of the device describedabove. This effect, and the improvement in multiplexibility of amatrix-type liquid crystal display of the twisted nematic kind, using amixture of this form, is further described in a copending UK PatentApplication.

In an alternative embodiment of the invention (cholesteric-to-nematic)phase change effect device incorporates a material as defined above.

A cell is prepared containing a long helical pitch cholesteric materialsandwiched between electrode-bearing glass slides as in the twistednematic cell described above. However the polarisers and surfacepreparations for homogeneous alignment, eg treatment of the glass slidesurfaces with SiO, are not used in this case.

If the glass slides are untreated and the liquid crystal material has apositive dielectric anisotropy (Δε) the liquid crystal material is in atwisted focal conic molecular texture in the OFF state which scatterslight. The effect of an electric field applied between a pair ofelectrodes on the respective inner surface of the glass slides is toconvert the region of liquid crystal material between the electrodesinto the ON state which is a homeotropic nematic texture which is lessscattering than the OFF state.

If the inner glass slide surfaces are treated, eg with a coating oflecithin, to give alignment perpendicular to those surfaces, and theliquid crystal material has Δε negative the material in the OFF state isin a homeotropic texture which has little scattering effect on incidentlight. If an electric field is applied between a pair of electrodes onthe respective inner surfaces of the glass slides the region of liquidcrystal material between the electrodes is converted to a twistedhomogeneous texture which scatters light (the ON state).

The contrast between the two states in each case may be enhanced by theaddition of a small amount of a suitable pleochroic dye (eg 1% by weightof 1,5-bis-4'n-butylphenylamino anthraquinone in the case where Δε ispositive) to the liquid crystal material.

A suitable positive dielectric anisotropy material embodying theinvention for use in a phase change effect device is:

    __________________________________________________________________________     Mixture B                                                                           ##STR32##                   60% by weight                              C15 (known)                                                                          ##STR33##                  10% by weight                                      ##STR34##                  30% by weight                               __________________________________________________________________________

A suitable negative dielectric anisotropy material embodying theinvention for use in a phase change effect device is:

    ______________________________________                                         ##STR35##               45% by weight                                         ##STR36##               46% by weight                                         ##STR37##               10% by weight                                         ##STR38##                1% by weight                                        (R.sub.C = (+)2-methylbutyl)                                                  ______________________________________                                    

The esters in the above device mixtures wherein X=H and Y=F may bereplaced by, or substituted partly in weight percentage by esterswherein Y=H and X=F (see Formula I above), eg ##STR39##

We claim:
 1. A liquid crystal ester compound having a molecularstructurewherein X is fluorine and Y is hydrogen or X is hydrogen and Yis fluorine, R₁ is an alkyl group and Z is ##STR40## where R₃ is analkyl group.
 2. A compound as claimed in claim 1 and wherein X ishydrogen and Y is fluorine.
 3. A compound as claimed in claim 1 andwherein X is fluorine and Y is hydrogen.
 4. A compound as claimed inclaim 2 and wherein R₁ is n--C₇ H₁₅ and R₃ is n--C₃ H₇.
 5. A compound asclaimed in claim 2 and wherein R₁ is n--C₅ H₁₁ and R₃ is n--C₄ H₉.
 6. Acompound as claimed in claim 2 and wherein R₁ is n--C₇ H₁₅ and R₃ isn--C₄ H₉.
 7. A compound as claimed in claim 2 and wherein R₁ is n--C₃ H₇and R₃ is n--C₅ H₁₁.
 8. A compound as claimed in claim 2 and wherein R₁is n--C₅ H₁₁ and R₃ is n--C₅ H₁₁.
 9. A compound as claimed in claim 2and wherein R₁ is n--C₇ H₁₅ and R₃ is n--C₅ H₁₁.
 10. A compound asclaimed in claim 2 and wherein R₁ is n--C₃ H₇ and R₃ is n--C₄ H₉.
 11. Acompound as claimed in claim 2 and wherein R₁ is n--C₄ H₉ and R₃ isn--C₅ H₁₁.
 12. A compound as claimed in claim 2 and wherein R₁ is n--C₆H₁₃ and R₃ is n--C₅ H₁₁.
 13. A compound as claimed in claim 2 andwherein R₁ is n--C₈ H₁₇ and R₃ is n--C₅ H₁₁.
 14. A compound as claimedin claim 2 and wherein R₁ is n--C₉ H₁₉ and R₃ is n--C₅ H₁₁.
 15. Acompound as claimed in claim 2 and wherein R₁ is (+)-2-methylbutyl andR₂ is n--C₅ H₁₁.
 16. A compound as claimed in claim 2 and wherein R₁ isn--C₃ H₇ and R₃ is n--C₆ H₁₃.
 17. A compound as claimed in claim 2 andwherein R₁ is n--C₅ H₁₁ and R₃ is n--C₆ H₁₃.
 18. A compound as claimedin claim 2 and wherein R₁ is n--C₆ H₁₃ and R₃ is n--C₆ H₁₃.
 19. Acompound as claimed in claim 2 and wherein R₁ is n--C₇ H₁₅ and R₃ isn--C₆ H₁₃.
 20. A compound as claimed in claim 2 and wherein R₁ is n--C₈H₁₇ and R₃ is n--C₆ H₁₃.
 21. A compound as claimed in claim 2 andwherein R₁ is n--C₉ H₁₉ and R₃ is n--C₆ H₁₃.
 22. A compound as claimedin claim 2 and wherein R₁ is n--C₃ H₇ and R₃ is n--C₇ H₁₅.
 23. Acompound as claimed in claim 2 and wherein R₁ is n--C₅ H₁₁ and R₃ isn--C₇ H₁₅.
 24. A compound as claimed in claim 2 and wherein R₁ is n--C₆H₁₃ and R₃ is n--C₇ H₁₅.
 25. A compound as claimed in claim 2 andwherein R₁ is n--C₇ H₁₅ and R₃ is n--C₇ H₁₅.
 26. A compound as claimedin claim 2 and wherein R₁ is n--C₈ H₁₇ and R₃ is n--C₇ H₁₅.
 27. Acompound as claimed in claim 2 and wherein R₁ is n--C₉ H₁₉ and R₃ isn--C₇ H₁₅.
 28. A compound as claimed in claim 2 and wherein R₁ is n--C₁₁H₂₃ and R₃ is n--C₇ H₁₅.
 29. A compound as claimed in claim 2 andwherein R₁ is (+)-2-methylbutyl and R₃ is n--C₇ H₁₅.
 30. A compound asclaimed in claim 2 and wherein R₁ is n--C₃ H₇ and R₃ is n--C₈ H₁₇.
 31. Acompound as claimed in claim 2 and wherein R₁ is n--C₅ H₁₁ and R₃ isn--C₈ H₁₇.
 32. A compound as claimed in claim 2 and wherein R₁ is n--C₇H₁₅ and R₃ is n--C₈ H₁₇.
 33. A compound as claimed in claim 2 andwherein R₁ is n--C₅ H₁₁ and R₃ is n--C₉ H₁₉.
 34. A compound as claimedin claim 2 and wherein R₁ is n--C₇ H₁₅ and R₃ is n--C₉ H₁₉.